This question is a little bit unrelated but I don't want to make a separate account on some aquarium website just to ask this. Hah. :)

I made the (probably unwise) decision to buy a koi a few months ago; I have a 25-gallon tank. He seems to be doing fine, except he started to get a few black/brown spots on some scales. Read this could be ammonia burn. Checked ammonia twice and it's at 0 ppm, but no guarantee the test was accurate. I put an ammonia removal fizz tab in the tank. I noticed more spots recently.

I realize putting him in a (heated) tank isn't really the best and I want to get some other fish so I'm going to try to find someone that wants him or, if I can't, bring him back to the store; they said they'd take it back when he gets too big. But I'm wondering... will the store possibly not accept him because of these spots? What about water quality? pH is fine, ammonia is apparently fine.

By the way.. I think he is a male. Extra points if you can identify the sex based on this picture (sorry; not the best picture for sexing). I'm not quite sure even after observing body shape, gonad region, fins, and fin transparency. I think it's too young to tell.

Hi Animus. The picture is a bit blurred, so can't be sure but they look a bit like argulus, or fish lice. If you look very closely they should be more or less the same rounded shape and be on top of the scales? Looking very close you can see little 'eyes'. If they are argulus you can hold the fish and pick them off with tweezers. Put them in water and feed with blood meal, and they might survive for a while. They swim around like little fish. cute! Treatment of the tank is as for any parasite infestation. You can buy treatment from any aquarium shop. You wouldn't want to be handling every fish in the tank!

Oh, wow that is interesting! And pretty creepy. I already gave the fish to my friend, who gave it to her neighbor because it didn't cope well with my friend's fish and would be better off in the neighbor's koi pond. So I can't check the fish and confirm that it's argulus.

It was more like the whole scale (or the visible part, anyway) was discolored. Didn't really check it out closely but definitely didn't look like a living organism. I want to visit my fishy so if I get the chance I'll see if he still has these discolorations and try to get a closer look. Erm.. it's the neighbor's problem for now I suppose, if it is lice. Oops.

The neighbours problem. Well yes, it was silly of the neighbour to put a fish in a pond without first quarantining it but I still feel sorry for them. They could have a very expensive cleaning program on their hands. Any new animals should always be quarantined and checked out, before adding them to existing stock. This should be standard practice for any zoologist. (I know, I have to plead guilty too. Don't we all?)